The present invention relates to a bulk material irradiation system and method, and more particularly to a system for transporting and irradiating bulk material in a manner such that a precisely controllable dose of irradiation is efficiently delivered to the material.
Irradiation technology for medical and food sterilization has been scientifically understood for many years dating back to the 1940's. The increasing concern for food safety as well as safe, effective medical sterilization has resulted in growing interest and recently expanded government regulatory approval of irradiation technology for these applications. United States Government regulatory agencies have recently approved the use of irradiation processing of red meat in general and ground meat in particular. Ground meat such as ground beef is of particular concern for risk of food borne illness due to the fact that contaminants introduced during processing may be mixed throughout the product including the extreme product interior which receives the least amount of heat during cooking. Irradiation provides a very effective means of reducing the population of such harmful pathogens.
The available sources of ionizing radiation for irradiation processing consist primarily of gamma sources, high energy electrons and x-ray radiation. The most common gamma source for irradiation purposes is radioactive cobalt 60 which is simple and effective but expensive and hazardous to handle, transport, store and use. For these reasons, electron beam and x-ray generation are becoming the preferred technologies for material irradiation. An exemplary maximum electron beam energy for irradiation purposes is on the order of 10 million electron-volts (MeV) which results in effective irradiation without causing surrounding materials to become radioactive. The necessary electron beam power must be on the order of 5 to 10 kilowatts or more to effectively expose materials at rates sufficient for industrial processing.
Electron beam and x-ray irradiation systems both employ an electron accelerator to either emit high velocity electrons directly for irradiation or to cause high velocity electrons to collide with a metal conversion plate which results in the emission of x-rays. A number of electron acceleration techniques have been developed over the past several decades including electrostatic acceleration, pumped cylindrical accelerators and linear accelerators.
Electrostatic accelerators are characterized by the use of a direct current static voltage of typically 30 to 90 kilovolts which accelerates electrons due to charge attraction. Electrostatic accelerators are limited in maximum energy by the physical ability to generate and manage high static voltage at high power levels. Electrostatic accelerators using Cockroft-Walton voltage multipliers are capable of energy levels of up to 1 MeV at high power levels, but the 10 MeV energy level utilized by many systems for effective irradiation is not typically available.
Various types of pumped cylindrical electron beam accelerators have been known and used for many years. These accelerators generally operate by injecting electrons into a cylindrical cavity, where they are accelerated by radio frequency energy pumped into the cylinder. Once the electrons reach a desired energy level, they are directed out of the cylinder toward a target.
RF linear accelerators have also generally been in use for many years and employ a series of cascaded microwave radio frequency tuned cavities. An electron source with direct current electrostatic acceleration injects electrons into the first of the cascaded tuned cavities. A very high energy radio frequency signal driven into the tuned cavities causes the electrons to be pulled into each tuned cavity by electromagnetic field attraction and boosted in velocity toward the exit of each tuned cavity. A series of such cascaded tuned cavities results in successive acceleration of electrons to velocities up to the 10 MeV level. The accelerated electrons are passed through a set of large electromagnets that shape and direct the beam of electrons toward the target to be irradiated.
A typical industrial irradiation system employs an electron beam accelerator of one of the types described, a subsystem to shape and direct the electron beam toward the target and a conveyor system to move the material to be irradiated through the beam. The actual beam size and shape may vary, but a typical beam form is an elliptical shape having a height of approximately 30 millimeters (mm) and a width of approximately 45 mm. The beam is magnetically deflected vertically by application of an appropriate current in the scan deflection electromagnets to cause the beam to traverse a selected vertical region. As material to be irradiated is moved by conveyor through the beam, the entire volume of product is exposed to the beam. The power of the beam, the rate at which the beam is scanned and the rate that the conveyor moves the product through the beam determines the irradiation dosage. Electron beam irradiation at the 10 MeV energy level is typically effective for processing of food materials up to about 3.5 inches in thickness with two-sided exposure. Conversion of the electron beam to x-ray irradiation is relatively inefficient but is effective for materials up to 18 inches or more with two-sided exposure.
The prior art industrial irradiation systems previously described are typically relatively inflexible and require careful setup, calibration and operation to deliver the irradiation dosage required for safe, effective sterilization. The output energy levels are established by the structure of the accelerator and are relatively constant. The output power levels are determined by equipment settings and calibration and may vary significantly.
Prior art irradiation systems of the direct electron beam type typically employ electron beam accelerators to generate a stream of electrons at energy levels of a maximum of 10 MeV. Scanning of the electron beam is performed using magnetic deflection similar to the type used for television raster scan. The dosage of irradiation delivered to a product passing by the accelerator is determined by the power of the beam, the beam scanning speed and the rate that the product is moved by the conveyor through the beam. This dosage is typically set manually by an operator for a given material to be irradiated, and is expected to remain constant at that setting. While this type of system can deliver effective radiation for a homogeneous product line, there are a number of shortcomings associated with the system. First, there are a number of factors that may cause the output power to vary after being set by the operator, including changes in temperature of critical components or shifting of frequency of the critical radio frequency acceleration drive subsystem. Second, it is cumbersome and inefficient to change the irradiation dosage to be delivered by the system if some different product is to be irradiated that requires different exposure. This characteristic of prior art systems generally dictates that the product mix to be irradiated can change very little during the course of processing. Third, there is no indication that irradiation exposure has been delivered to the products. Physical dosimeters must be placed periodically on the conveyor or within packages of products and examined to determine that products have indeed been irradiated at the specified dosage. Until the dosimeters have been verified, all product that has passed through the irradiation system must be held in quarantine awaiting verification that the processing was successful. If there is a failure indicated by an underexposed trailing dosimeter, all of the product that is held in quarantine is of unknown status, with some amount at the front of the batch probably exposed and some amount at the back of the batch probably unexposed. Depending on the severity of the unknown product irradiation implications, the entire batch may have to be destroyed.
A conveyor-based irradiation system that addresses many of the shortcomings of prior art systems is disclosed in U.S. application Ser. No. 09/685,779 filed Oct. 10, 2000 for “Irradiation System And Method” by S. Lyons, S. Koenck, B. Dalziel and J. Kewley, which is hereby incorporated by reference. Improvements in the state of the art may also be achieved in a bulk material irradiation system, which is the subject of the present invention.